Endogenous inflammatory factors are key to the pathogenesis of inflammatory and autoimmune disorders such as rheumatoid arthritis and atherosclerosis. Acute-phase proteins (APPs) are produced in response to infectious agents as well as non-infectious insult such as trauma, burn, surgery and malignancy. Clinical evidence suggests that APPs such as serum amyloid A (SAA) and C-reactive proteins (CRP) are major biomarkers for inflammatory diseases. However, the relationship between APP production and disease progression remains undefined. Our recent work shows that SAA has cytokine-inducing activity and is an endogenous stimulant for IL-23 expression. We also identified Toll- like receptor 2 (TLR2) as a functional receptor for SAA, and other published reports suggest that SAA activates TLR4 and the chemoattractant receptor FPR2. Given recent implication of these receptors in inflammatory and autoimmune diseases, we hypothesize that SAA is an endogenous mediator that deciphers danger signals through activation of the TLRs and chemoattractant receptors. In this application, we will establish an active role of SAA in sterile inflammation and identify the respective functions of the SAA receptors. Aim 1 will examine the effect of global rise in SAA concentration by investigating a potential role of HDL in modulating the cytokine-inducing activity of SAA, which is present in the blood circulation as an apolipoprotein of HDL. We will determine how the presence of HDL influences the in vivo functions of SAA by comparing the activity of SAA in WT and apo A-1-/- apo E-/- double KO mice, which lack HDL. Using a mouse model recently developed in our lab, we will examine the effect of HDL on the ability of SAA to induced neutrophilia in mice. In Aim 2, we will determine the structural and functional properties of SAA isoforms in activating the SAA receptors (TLR2, TLR4 and FPR2). Using SAA receptor reconstitution and DNA mutagenesis, we will identify structural determinants for SAA interaction with TLR2, TLR4 and FPR2, thereby defining the specific SAA isoforms and receptors with respect to their biological functions. Characterization of the mouse SAA3 will be conducted to explore mSAA3 as a non-hepatic APP that resembles locally produced SAA in human diseases. In Aim 3, we will determine the pathophysiological functions of SAA in an inflammatory disease model. Using SAA transgenic mice, we will examine in vivo SAA expression in the development and progression of inflammatory arthritis. The involvement of the SAA receptors in joint inflammation will be determined using knockout mice lacking the individual receptors. Collectively, these studies aim to establish SAA as an important endogenous ligand and a damage-associated molecular pattern for activation of inflammatory cells through TLRs and chemoattractant receptors.